1. Field of the Invention
This invention relates to a process for drying water-containing useful materials or mixtures thereof, more particularly with high sensitivity to heat and preferably with a suitability as--or for use in--wetting agents, detergents and/or cleaning products, in a vertical tower using superheated steam circulating in countercurrent to the descending material to be dried as the drying gas.
2. Discussion of Related Art
The drying of aqueous preparations of useful materials and mixtures of useful materials suitable as wetting agents, detergents and/or cleaning products with superheated steam as the drying gas is known from Applicants' International patent application WO 92/5849. The advantages of this method of drying over conventional spray drying using hot air as the drying gas lie on the one hand in the absence of oxygen so that the problems involved in the drying of purely organic or substantially organic useful materials, for example corresponding surfactants based on natural materials, for example oxidative damage or the risk of fire, are avoided. Circulation of the drying gas provides for operation in the virtual absence of waste gases. Other advantages attributable inter alia to the fundamental differences in the drying gas can be found in the cited document.
Circulation of the drying gas to enable the drying process to be carried out in the absence of waste gases means that the installation has to be closed. The presence of extraneous air in conventional spray drying with hot air or hot combustion gases as the drying gas would not be possible here or at least would be inappropriate. Accordingly, the closed construction of the drying installation operated with superheated steam leads to a virtually flow-free space below the lowermost superheated steam inlets, inside which the temperature substantially corresponds to the entry temperature of the superheated steam. The walls in the lower part of the drying tower also have this temperature unless special precautions are taken.
Numerous products intended for drying, more particularly organic products, have tacky properties after drying or at least pass through an adhesive phase during the drying process. Accordingly, the completely or partly dried particles cake on the one hand together and, on the other hand, on the inner wall of the tower in the substantially flow-free space situated beneath the superheated steam inlets. This effect is intensified by the relatively long residence time of the particles in that space. Thermal damage to the particles caking on the hot walls of the tower is unavoidable.
In conventional spray drying using hot air as the drying gas, the problem is solved by rapid discharge of the product through a permanently open channel at the lower end of the drying tower. This solution is not applicable to drying with a circulated drying gas which is carried out in the substantial absence of waste gases.
The problems caused by the caking of thermoplastic materials and sugar-containing foods on the tower walls of spray dryers is known from the prior art (K. Kroll, Trocknungstechnik, Vol. 2, Trockner und Trocknungsverfahren (Dryers and Drying Processes), 2nd Edition 1978, Springer Verlag, page 294). Materials of this type remain soft and tacky towards the end of drying even at moderately high temperatures. To solve this problem, the walls of the dryers used for such materials are provided with air-permeated casings or, alternatively, cleaning jets are allowed to rotate inside the drying tower and blow away the powder with cold air. It is also known that a cool and moisture-free airstream can be introduced into the dryer in such a way that it sweeps along the inside of the walls in the form of a downwardly directed curtain of cold air. The introduction of cold air or a curtain of cold air takes place at the lower end of the cylindrical drying tower, especially for the normally conical discharge funnel. The cold air is introduced into the interior of a double jacket through a ring channel arranged in the attachment zone of the cone and an air distributor plate, the inner wall of the double jacket terminating below the inlet substantially at a level where the inner and outer walls come very close to one another. The cold air flows through the narrow annular gap and is forced by this arrangement to flow tangentially along the inner wall of the cone.
Another example of the introduction of cold air into the discharge cone of a spray drying plant can be found in K. Kroll, Trocknungstechnik, Vol. 3, "Trocken und Trockner in Produktion (Drying and Dryers in Production)" by W. Kast et al., Springer Verlag 1989, page 188, FIG. 3.20. Coffee extract is dried in this spray drying tower. In this case, too, the cold air is introduced into the cone through a ring channel. Unfortunately, there are no further particulars in this document regarding the introduction of the cold air.
The introduction of cold air, even where it is made to float tangentially along the inner wall of the cone, is sufficient for conventional spray dryers operated with hot air as the drying medium. Dryers of this type normally work with an open discharge cone and, particularly in the spray drying of detergents and detergent ingredients, under a light reduced pressure to avoid the emission of dust to the outside. The extraneous air which thus flows into the drying tower from below has a whirling effect which prevents the development of excessive temperatures in the lower part of the spray dryer. In addition, the open discharge cone enables the dried product to be rapidly discharged so that its residence times in the lower part of the tower are brief.
Under these conditions, the above-mentioned injection of cold air into the discharge cone is sufficient to reduce the caking of soft and tacky products on the walls to such an extent that the product does not suffer unacceptable thermal damage. However, in the case of drying with circulated superheated steam under a light excess pressure, the known measures do not meet the requirement of ruling out substantial damage to the material being dried. High quality standards, particularly in the production of foods and detergents, are crucial in this regard.
Accordingly, in a drying process using superheated and circulated steam as the drying gas, the problem addressed by the present invention was to prevent the product from caking on the inner walls of the tower with an excessive residence time so that the product would not suffer any thermal damage.